Electrodes and battery cells

ABSTRACT

Ceramic electrodes of cadmium germanate and other ternary oxide materials, provide electrodes for secondary battery cells. Open circuit voltages of 1.5 volts are attainable. The absence of crystallographic phase changes on charge/discharge is noted and renders possible the use of solid electrolytes, possibly in a unitary ceramic structure.

BACKGROUND OF THE INVENTION

This invention relates to electrodes for use in batteries, and inrelated applications. The invention also relates to battery cells.

The commonly employed Ni/Cd secondary battery cell has a number ofimportant disadvantages. The open cell voltage (typically around 1.15volts) is too low for the battery to be conveniently interchangeablewith primary batteries having an open cell voltage of around 1.5 volts.Both forms of cadmium appearing in the battery--metallic cadmium andcadmium oxide--are known to be toxic and the standard electrolyte ofconcentrated potassium hydroxide is extremely corrosive. For thesereasons, guaranteed containment of the cell contents is essential andthe production costs of the cell housing are relatively high.

The number of charge/discharge cycles that can reliably be undergone bya Ni/Cd battery may be enough for some domestic purposes but is notsufficient for many specialised applications where a battery is requiredto undergo reliably 1,000; 5,000 or even more charge/discharge cycles.An important factor in limiting the charge/discharge life of a Ni/Cdbattery is the internal stress created upon the change of phase frommetallic cadmium to cadmium oxide. Each charge/discharge cycle involvesa structural change within the cadmium electrode resulting, over a fewhundred cycles, in mechanical deterioration. The same factor will applyto a number of other known battery electrodes such as MnO₂, whichinvolve a change of phase.

In cases where there is a limited supply of charging energy, it isnecessary to take into account the energy associated with the phasechange. When used, for example, as a storage battery in combination withsolar cells, the Ni/Cd battery is able to make less use of the solarcharging energy than would be the case if no phase change were involved.

It should further be noted that cadmium oxide, in common with a numberof other known electrode materials, is insufficiently electricallyconductive to be used directly as an electrode and requires to be mixedwith graphite in production of a battery cell. This may involvemanufacturing difficulties. The need to incorporate the material withinan electrode which is not inherently electroactive is more generallyregarded as an important drawback.

Reference is directed to Journal of the Electrochemical Society July1987, pages 1591-1594 Hashemi et al "Dicadmium Stannate as a NovelElectrode Material for Battery Applications". This proposes an electrodefor secondary battery cells which offers some advantages over theconventional electrodes.

It is one object of the present invention to provide a further improvedelectrode which can be used in a secondary battery cell substantially toovercome some or all of the described disadvantages of known secondarybattery cells.

SUMMARY OF THE INVENTION

Accordingly, the present invention consists in one aspect in anelectrode for use in battery cells or the like, comprising cadmiumgermanate as the electrode material.

Preferably, the electrode comprises a sintered body of cadmiumgermanate.

In another form, the present invention consists in a rechargeablebattery having a negative electrode comprising cadmium germanate as theelectroactive material.

Advantageously, the negative electrode comprises a porous ceramic bodyof cadmium germanate.

A rechargeable battery cell using cadmium germanate as the electroactivematerial for the negative electrode offers a number of advantages overcommercially available batteries.

There is no substantial structural difference between the oxidised andreduced states of the Cd₂ GeO₄ electrode and hence the charge/dischargecycle is not associated with any phase change. This is in contrast withknown electrodes such as CdO and MnO₂. As has been mentioned above, thelack of a phase change is important for two reasons. First, the energynormally associated with this phase change (in--for example--Ni/Cdbatteries) no longer needs to be considered within the charge/dischargecycle. The energy balance within the cycle becomes increasinglyimportant when the battery is used in applications where there is alimited supply of energy, such as in the storage of solar energy inconjunction with solar cells. A battery constructed using a Cd₂ GeO₄electrode is clearly able to store much more of the energy than aconventional rechargeable battery. Secondly, the phase transformationtaking place in other electrode systems during the charge/dischargecycle can lead to the build up of internal stresses, limiting the numberof cycles available in conventional rechargeable batteries. Since nophase change takes place with the Cd₂ GeO₄ electrode, the number ofcharge/discharge cycles is greatly increased.

In a further aspect, the present invention consists in a rechargeablebattery comprising a positive electrode, an electrolyte and a negativeelectrode comprising a ceramic electrode body comprising a ternary oxidematerial of formula A_(x) BO_(y), where A and B are ions of differentmetals; x=1 or 2 and y=2, 3 or 4, the negative electrode undergoing oncharging and discharging off the battery cell an electrochemicalreaction involving no change of phase in A or B and the battery cellproviding an open cell voltage of at least 1.5 volts.

Preferably, the ternary oxide material is selected From the groupconsisting of zinc stannate, lead stannate, barium stannate, magnesiumstannate, barium germanate and lead germanate.

It is found, when used for example with the same positive electrode as aconventional Ni/Cd battery, at room temperature, the electrodesaccording to this preferred form of the invention are capable ofproviding an open circuit voltage of at least 1.5 volts. This means thatbatteries utilising these electrodes can be used as one-for-onereplacements for conventional primary battery cells.

Cadmium germanate and the other ternary oxide materials that have beenspecifically mentioned can be processed using conventional ceramicpowder techniques enabling complex shapes to be produced without costlymachining operations. In addition, the surface area of the electrode canbe controlled by means of a choice of porosity, enabling the formationof large surface areas within a small volume, if required. This meansthat miniature batteries having a high open circuit voltage or batteriesmanufactured with electrodes of highly specialised shape, may bepossible. Since the materials can also be evaporated, it should bepossible to use thick film techniques in the production of electrodes.

The electrical conductivity of cadmium germanate and zinc stannate issufficiently high to enable the formation of an electrode withoutgraphite or other conductive filler material. This may enablemanufacture to be simplified and electrodes to be reduced in volume.

The choice of electrolyte is of course an important aspect of batterydesign. Commonly, liquid electrolytes are employed, but for low powerapplications requiring long shelf and service life, solid-electrolytebattery cells have been developed. Solid-state lithium battery cells arecommercially available and used for applications such as heartpacemakers and within computers to preserve volatile memory. Lithiumbattery cells are, however, limited to low power applications (typicallymicrowatts). This is because of the high impedance of most solid stateelectrolytes at normal ambient temperatures, high contact resistancebetween electrodes and electrolyte and the possibility of mechanicalstress through volume changes associated with temperature or withelectrode discharge reactions.

A known, improved solid-state battery cell uses a solid electrolyte ofrubidium silver iodide which exhibits an unusually high ionicconductivity of 0.26 (ohm.cm)⁻¹ at room temperature. A silver anode isemployed with the cathode formed as a mixture of carbon andtetramethylammonium pentaiodide (Me₄ NI₅). It emerges, however, thathigh electrolyte conductivity does not itself overcome all the problemsassociated with solid-state battery cells. Electrode/electrolyteinterfacial resistance must also be taken into account in this respectand, in contrast to the more familiar situation with conventionalaqueous systems, where the solid electrodes are uniformally wetted bythe liquid electrolyte, the solid state configuration of the cell maycreate non-uniform contact at the interface. Differential expansion andcontraction of electrodes and electrolyte may lead to poor contact andconsequential high internal resistance. This situation is undesirable inprimary battery cells but becomes an extremely serious problem withsecondary cells. Here, the creation of localised areas of contactbetween electrode and electrolyte promotes, on charging and discharging,localised deposition of metal in dendritic Form. The problem is thusaggravated and large metallic depositions can indeed lead to fracture.

Interfacial polarization phenomena are presently among the most severeproblems in the development of practical secondary cells. Some work hasrecently been done to reduce this problem by mixing the electrodematerial with the electrolyte, for example as compressed powders, toform an electrodic mass with an enlarged interfacial area. In this way,the current density at the electrode/electrolyte interface is reducedand the problems of polarization are, it is hoped, alleviated. Thereare, however, serious problems in the development of such cells with theresult that practical solid-state cells capable of operating at ambienttemperatures remain of the primary type.

It is an object of a still further Form of the present invention toprovide an improved rechargeable battery cell with a solid electrolyte.

Accordingly, in yet a further aspect, the present invention consists ina rechargeable battery comprising a positive electrode, a solidelectrolyte and a negative electrode comprising a ceramic electrode bodycomprising a ternary oxide material of formula A_(x) BO_(y) where A andB are ions of different metals, x=1 or 2 and y=2,3 or 4, the negativeelectrode undergoing on charging and discharging of the battery andelectrode chemical reaction involving no change of phase in A or B.

Preferably the ternary oxide material is selected from the groupconsisting of cadmium germanate, cadmium stannate, zinc stannate, bariumstannate, magnesium stannate, barium germanate, magnesium germanate andlead germanate.

It will be understood that because the reduction of the ternary oxidematerial in the charge cycle results in the formation of lower valencystates of oxides but not the metallic phase, the problem of localisedmetallic deposition at the electrode/electrolyte interface is removed.Since, as explained above, there is no significant crystallographicphase change associated with charge and discharge, the interfacialstresses are very much reduced leading to a considerable improvement inthe number of charge/discharge cycles that can be expected. The use of aceramic electrode body with a solid electrolyte will be expected also toreduce problems of differential thermal expansion.

A wide variety of solid electrolyte materials can be employed in thepresent invention, including rubidium silver iodide as mentioned above,or β aluminas of the general formula AM₁₁ O₁₇ where A is Na, K, Rb, Ag,Te, or Li and M is Al, Fe or Ga.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a cyclic voltammogram of an electrode according to the presentinvention, and

FIG. 2 is a diagrammatic representation of a battery cell according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the production of a cadmium germanate electrode according to thisinvention, pellets of 2Cd0:1GeO₂ were presintered at 850° C. for 12hours, after which they were crushed and ground. Pellets were thenrepressed and sintered at 1080° C. for a further 12 hours. The sinteringprocess yielded a body with approximately 40% porosity which isfavourable for electrochemical applications since the surface area isextremely high. An ohmic contact was applied to one side of the pelletwhich was then covered with epoxy resin so as to expose one face only tothe electrolyte.

A simple rechargeable cell was constructed using the above describedcadmium germanate electrode, without further treatment, as the negativeelectrode. The positive electrode was prepared From a commercial Ni/Cdcell having a theoretical maximum energy density of 320 mAh/g. Theelectrolyte was a dilute solution of NaOH and the electrodes weremounted in a simple beaker-type glass cell with no separator. Chargingwas conducted galvanostatically and discharging was carried out under aconstant load condition. In order to investigate the charge to dischargecharacteristics of the batteries forced polarization was performed withan external current off 10 mA. After each cycle, the coulombicefficiency was determined and compared with data from previous cycles.No significant changes of efficiency were observed after severalthousand charge/discharge cycles. It will be recognized that repeated,forced polarization is a particularly harsh test of an electrode andeven longer lives would be expected under "normal" charge/dischargeconditions.

Self discharge and stability of the constructed cells were investigatedby charging a cell and monitoring its open circuit voltage over a periodof time. After an initial, small voltage drop which was anticipated fromthe behavior of known cells, the cell according to this inventionexhibited an open circuit voltage of 1.45 volts for a period of time inexcess of four months.

To investigate the electrochemical reactions, a cyclic voltammogram ofthe cadmium germanate electrode was produced using an auxiliaryelectrode of platinum and a saturated calomel reference electrode. Thesweeping rate was 250 mV/min and the cells operated at 25° C. From theaccompanying figure, it can be seen that there are two major waves ineach branch of cathodic and anodic voltammograms. These waves wereabsent in voltammograms of Pt/Pt in identical conditions. It can beconcluded that the waves on the catbodie branch are due to the reductionof two species in the working electrode, Ge and Cd. Conversely, the tworeduction waves are replaced in the anodic branch by oxidation waves,implying that both Ge and Cd return to their original (as sintered)valency states.

X-ray diffraction patterns of both reduced and oxidised samples wereidentical, suggesting that no substantial structural changes take placewith either the cathodic or anodic polarisation. Moreover, no sign ofmetal formation was observed during the reduction cycle, even when theapplied cathodic overpotentials were increased far beyond those ofhydrogen evolution. This is an important feature, indicating no need fora control system for the applied potentials, providing that they aregreater than the reduction potentials of Cd and Ge, in this case about-800 mV versus a saturated calomel electrode. This is in contrast withother techniques, for instance annealing the material in a reducingatmosphere where a complete reduction to tile metallic state is readilyattainable.

It is anticipated that the oxidation/reduction of cadmium germanate willinvolve minor changes in unit cell dimensions but these will beassociated with stresses and energy levels significantly below thoseencountered in a phase change.

The nominal capacity of the cadmium germanate electrode wasexperimentally determined using the chronoamperometry technique to be ofthe order of 210 mAh/g. This suggests that the Ge(IV)-Ge(II) transitionis not the only electrochemical reaction taking place during thecharge-discharge cycle, since GeO₂ comprises approximately 29 molar % ofthe electrode. Were this to be the only transition involved, thecorresponding theoretical capacity would be approximately 148 mAh/g. Thehigh value of measured capacity implies that during the charge cycle(reduction of the working electrode), some reduction of Cd(II) to Cd(I)takes place in addition to the Ge(IV)-Ge(II) reaction. The existence oftwo waves in each branch of the cyclic voltammogram discussed earlier,also supports this conclusion.

Turning to alternative ternary oxide materials, the open circuitvoltages in battery cells at room temperature are as follows:

    ______________________________________                                        Zinc Stannate      Zn.sub.2 SnO.sub.4                                                                     1.75 V                                            Barium Stannate    BaSnO.sub.3                                                                            1.60 V                                            Magnesium Stannate MgSnO.sub.3                                                                            1.55 V                                            Barium Germanate   BaGeO.sub.3                                                                            1.65 V                                            Magnesium Germanate                                                                              MgGeO.sub.3                                                                            1.60 V                                            Lead Germanate     PbGeO.sub.3                                                                            1.50 V                                            ______________________________________                                    

These alternatives materials share with cadmium germanate thecharacteristics that no phase change takes place during thecharge/discharge cycle (there being indentical X-ray diffractograms inboth oxidised and reduced forms) and that no further reduction to--forexample--the metallic state takes place on application of increasedcathodic current.

In a working battery cell according to this invention, it will bepossible to achieve an open circuit voltage of 1.5 volts together with asufficiently low internal resistance to enable one-for-one replacementof commercially available primary cells. The choice of electrolyte islargely unrestricted and a wide variety of dilute alkyline solutions canbe employed. The containment difficulties associated with corrosiveelectrolytes are thus avoided. The electrolyte can be chosen to suit themanufacturing technique employed and can be gelled so as to be heldwithin the porous electrode without Further containment. The electrodeacccording to this invention can be used with a variety of positiveelectrodes, the positive electrode of the conventional Ni/Cd batterybeing a convenient example.

The alternate has been mentioned above, of solid state electrolytes anda battery cell construction having a solid electrolyte will now bedescribed with reference to FIG. 2.

Using cadmium germanate or zinc stannate, negative electrode leaves 10and 12 are produced in "green" form, that is to say with the finalsintering step omitted. A current collector 14, taking the form, as anexample, of a tantalum grid, is sandwiched between the leaves 10 and 12.In similar fashion, positive electrode leaves 16, 18 are produced ofnickel oxide in "green" form, sandwiching a current collector 20.

An electrolyte block 22 is Formed of "green" rubidium silver iodide andpositioned between the two electrodes. Pressure is then applied in thelongitudinal direction of the stack, and a final sintering processundergone at around 1,000° C.

The effect of sintering the pressurised stack is to produce intimatebonding between each electrode and the electrolyte, with the currentcollectors becoming embedded within the respective electrodes. Becauseof the feature in electrodes according to this aspect of the invention,that there is no significant crystallographic phase change associatedwith charge and discharge, interfacial stresses in the solid statesbattery are very much reduced. The problem of localised metallicdeposition at the interfaces is removed, and a large number ofcharge/discharge cycles can be undergone without mechanical orelectrical deterioration. The unitary ceramic construction is robust andproblems of differential thermal expansion are unlikely to be serious.

It should be noted that whilst the planar stack geometry has the meritof simplicity of construction it is not the only alternative.

In addition to the inorganic solid electrolytes that have beendiscussed, there are a wide range of conductive polymers that can beemployed. Similarly, other positive electrode structures can be used.The manner of construction will then be selected to suit the particularchoice of electrolyte and positive electrode.

It should be understood that this invention has been described by way ofexample only, and a variety of further modifications are possiblewithout departing from the scope of the invention.

It is claimed:
 1. A rechargeable battery comprising a positiveelectrode, a solid electrolyte and a negative electrode comprising aceramic electrode body comprising a ternary oxide material, the negativeelectrode undergoing on charging and discharging of the battery anelectrode chemical reaction involving no change of phase, wherein theternary oxide material is selected from the group consisting of cadmiumgermanate, zinc stannate, barium stannate, magnesium stannate, bariumgermanate, magnesium germanate and lead germanate.
 2. A batteryaccording to claim 1 wherein the solid electrolyte is selected from thegroup consisting of silver rubidium iodide and β alumina.
 3. A batteryaccording to claim 1, wherein the ternary oxide material is cadmiumstannate.
 4. A battery according to claim 1, wherein the ternary oxidematerial comprises zinc stannate.
 5. A rechargeable battery comprising apositive electrode, an electrolyte and a negative electrode comprising aceramic electrode body comprising a ternary oxide material, the negativeelectrode undergoing on charging and discharging of the battery anelectrode chemical reaction involving no change in crystallographicstructure phase and the battery providing an open cell voltage of atleast 1.50 volts, wherein the ternary oxide material is selected fromthe group consisting of zinc stannate, lead stannate, barium stannate,magnesium stannate, barium germanate, and lead germanate.
 6. A batteryaccording to claim 5, wherein the ternary oxide material comprises zincstannate.
 7. A rechargeable battery comprising a positive electrode, anelectrolyte and a negative electrode, the negative electrode having aselectroactive material a ternary oxide selecting from the groupconsisting of cadmium germanate, zinc stannate, lead stannate, bariumstannate, magnesium stannate, magnesium germanate, barium germanate andlead germanate.
 8. A battery according to claim 7, wherein the ternaryoxide material comprises zinc stannate.
 9. A rechargeable batterycomprising a positive electrode, an electrolyte and a negative electrodecomprising a ceramic electrode body comprising a ternary oxide material,wherein the ternary oxide material is selected from the group consistingof cadmium germanate, zinc stannate, barium stannate, magnesiumstannate, barium germanate, magnesium germanate and lead germanate. 10.A rechargeable battery according to claim 9 wherein the negativeelectrode undergoes an electrochemical reaction with no change of phasein ionic components of the ternary oxides.
 11. A rechargeable batteryaccording to claim 9 comprising a positive electrode, a solidelectrolyte and a negative electrode comprising a ternary oxidematerial, wherein the solid electrolyte is a solid selected from thegroup consisting of silver rubidium iodide and β-alumina.
 12. A batteryaccording to claim 11, wherein the ternary oxide material is selectedfrom the group consisting of zinc stannate, lead stannate, bariumstannate, magnesium stannate, barium germanate and lead germanate.
 13. Abattery according to claim 12 wherein the electrolyte is a solid.
 14. Abattery according to claim 11, wherein the ternary oxide material isprovided in a porous ceramic body.
 15. A battery according to claim 14wherein the pores of said ceramic body contain electrolyte.
 16. Abattery according to claim 11, wherein the ternary oxide material iscadmium stannate.
 17. A battery according to claim 11, wherein theternary oxide material comprises zinc stannate.
 18. A battery accordingto claim 5 or claim 7, wherein the ternary oxide material is provided inthe form of a porous ceramic body.
 19. A battery according to claim 18wherein the pores of said ceramic body contain said electrolyte.
 20. Abattery according to claim 1 or claim 2 wherein the negative electrodeand solid electrolyte are sintered to form a unitary body.
 21. A batteryaccording to claim 20, wherein the negative electrode, the positiveelectrode and the solid electrolyte are sintered to form a unitaryceramic body.
 22. A battery according to any one of claims 9 and 11,wherein the negative electrode and solid electrolyte form a unitaryceramic body.
 23. A battery according to claim 22, wherein the negativeelectrode, the positive electrode and the solid electrolyte are sinteredform a unitary ceramic body.